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Re: [TowerTalk] M2 20M5 SWR measurements

To: "Towertalk" <>
Subject: Re: [TowerTalk] M2 20M5 SWR measurements
From: "Tony Brock-Fisher" <>
Date: Fri, 30 Dec 2011 22:55:26 -0500
List-post: <">>

Assuming your 8.35 foot jumper is lossless, it should not affect the SWR on the 
line. The addition of transmission line changes the complex impedance, but not 
the SWR. A perfect SWR meter would not show any difference with the addition of 
a length of lossless transmission line. A vector impedance meter of course 
would see a difference. For example, as transmission line is added, a Smith 
chart would show the different points along the line as a circle, having 
different real and complex impedances, but having a consistent SWR.

So what you are seeing is changes in the reported SWR with the two meters you 
have due to impedance changes. This can be for one of two (or more) reasons:

1. There is RF on the outside of the coax. Try adding several beads on the coax 
and see if this makes things calm down, and gets identical readings with and 
without the jumper. Many SWR meters can be affected by RF on the outside of the 

2. The meters are sensitive to the load impedance, and are not 'perfect' or 
'ideal' SWR meters. Perhaps try a Bird meter.

All of this is not really saying anything about the antenna itself. But you 
have to satisfy yourself that you are accurately measuring SWR before you can 
worry about what is happening at the antenna.


-Tony, K1KP

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